It is understood that there are methods and devices for determining compositions of fuel mixtures. For example, fuel mixtures are increasingly used in motor vehicles which are able to handle an admixture of ethanol and/or other alcohols, in addition to the proper mineral oil fuels. Flex-fuel vehicles which may be operated with variable ethanol/gasoline mixtures are known. The parameters of the engine control unit of the motor vehicle are adapted to the composition of each fuel mixture. Different concepts are in effect in different areas of the world. While in the U.S., usually only the essential adjustments of the engine control unit to the fuel mixture are made so far in order to be able to offer flex-fuel vehicles at all, efficiency and performance improvements are typically sought in Europe. In particular, however, the latter usually presupposes a precise knowledge of the composition of the fuel mixture, in particular an ethanol-fuel mixing ratio, in order to determine the optimal engine control parameters.
The ethanol-fuel mixing ratio is usually determined on the basis of available measured variables using software in the control unit itself, or this mixing ratio may be detected using an ethanol sensor. Such ethanol sensors may be based on numerous different measuring principles. Capacity measuring methods based on permittivity and conductivity determination are used in particular. As a rule, the permittivity of the fuel mixture is determined at frequencies up to approximately 1 MHz. However, one disadvantage of a simple permittivity determination (with temperature correction) at frequencies up to approximately 1 MHz is that in principle only the precise determination of the composition of mixtures of at most two components is possible when using these methods. This method cannot be used for detecting additional components of a fuel mixture in general.
To identify additional components such as H2O, for example, measurements in the GHz range are necessary because the permittivity of alcohol, water and other polar components, e.g., interfering components, drops sharply here with an increase in frequency due to the orientation polarization. One example of such methods which operate in the GHz range is described in DE 34 12 704 A1. A device for measuring the alcohol content in a fuel mixture which may be used in a fuel line is used. The fuel line is made of a material which is permeable for high-frequency signals. A microwave chamber is situated outside of the fuel line, enclosing a portion of this fuel line. The microwave chamber has a pair of waveguides situated opposite one another with the fuel line between them. One of the waveguides is provided with an antenna section for transmission of microwaves from a microwave generator. The other microwave guide is provided with a receiving antenna section to receive microwaves passing through the fuel line. Microwaves received by the antenna section are detected by a detector and converted into d.c. voltage signals, which correspond to the strength of the received microwaves.
There are two different approaches for control of engines having a fuel composition varying from pure gasoline to a gasoline-ethanol mixture containing approximately 85% ethanol for flex-fuel systems. Using software-based systems, the deviation and the actual value of the λ sensor signal from the setpoint value of the applied mixture pre-control is observed. If this deviation occurs after a detected tank filling operation, then a change in the ethanol content is inferred. On the other hand, in sensor-based systems, the ethanol content of the fuel is measured directly using an ethanol sensor, usually located in the fuel supply line.
In both systems, the ignition angle is adjusted based on the engine control unit manipulated variables, for example, the fuel metering, as a function of the detected ethanol content.
The less expensive software-based systems have become popular in Brazil, which is today the most important market for flex-fuel vehicles. The European Union and NAFTA, which will become more important markets in the future, are being influenced by discussions by the vehicle manufacturers referring to the need for an ethanol sensor, i.e., a sensor-based system for detecting the composition of the fuel mixture. The strict OBD-II requirements (on-board diagnostics of the second generation) usually argue for sensor-based systems. A rapid and unambiguous allocation of a mixture deviation to the error source “error in fuel system” or to an altered fuel composition is presumably impossible to perform without using an ethanol sensor. In the case of saddle tanks, such as those described in DE 10 2007 039861 A1, for example, two interconnected tank chambers are provided. The saddle tank design is based on the fact that space must be provided for the drive shaft and the differential gear at the bottom of the tank in vehicles with rear-wheel drive. When filling the tank with smaller quantities, it may happen with saddle tanks that the tank quantity is stored in only one of the two tank chambers. If, for example, one of the tank chambers is filled with fuel grade E 85 and the second tank chamber is filled with E 110, a relatively rapid change in fuel composition will occur while driving when the fuel supply is switched from one tank chamber to the other tank chamber.